Define "cheap". What's "cheap" to one person may not be to another.
Define the parameters. If you're using it to supplement iron sights at shorter ranges, something will be better than nothing. If you're using it with a magnified optic, you may be better off using your riflescope.
Without the above being answered, it's hard to give a good answer. The primary purpose of a spotting scope is to spot, so resolution better be a high priority with the lens quality, which doesn't come cheap - I don't care what anybody says.
I've had the B&L Elite 15-45x and thought it was a great spotter for the dollar. It's now marketed under Bushnell. The Army issued it as the M144 (I think), but they're about $350. Out of all I've used, I haven't used much less that I've been impressed with.